Gamma rays are high-energy subatomic particles formed either by the decay of radioactive elements or by nuclear reactions. The wavelength of a gamma ray is very short—less than the radius of an atom—and the energy they carry can measure millions of electron volts.
Terrestrial gamma rays—those produced on Earth—are the only gamma rays we can observe here. A second class of gamma rays, called cosmic gamma rays, do not penetrate to the surface of Earth because the ozone layer absorbs high-energy radiation. The only way to detect cosmic gamma rays, which are created by nuclear fusion reactions that occur within the core of stars, is by sending a satellite-observatory into space.
Cosmic gamma rays were first discovered in 1967 by small satellites called Velas. These military satellites had been put into orbit to monitor nuclear weapon explosions on Earth, but they found gamma ray bursts from outside our solar system as well.
Several other small satellites launched in the early 1970s gave pictures of the whole gamma-ray sky. These pictures reveal hundreds of previously unknown stars and several possible black holes, the remains of massive stars. Thousands more stars were discovered in 1977 and 1979 by three large satellites called High Energy Astrophysical Observatories. They found that the entire Milky Way galaxy shines with gamma rays.
Then, on April 5, 1991, the National Aeronautics and Space Administration (NASA) sent the Compton Gamma Ray Observatory (CGRO) into space aboard the space shuttle Atlantis. During its nine-year mission, this 17-ton (15.4-metric ton) observatory provided scientists with an all-sky map of cosmic gamma-ray emissions, as well as new information about supernovas, young star clusters, pulsars, black holes, quasars, solar flares, and gamma-ray bursts. Gamma-ray bursts are intense flashes of gamma rays that occur uniformly across the sky and are of unknown origin. The energy of just one of these bursts has been calculated to be more than 1,000 times the energy that our Sun will generate in its entire ten-billion-year lifetime.
Antimatter: Matter composed of antiparticles, or subatomic particles identical to one another in mass but opposite in electric and magnetic charge. When an electron (with a negative charge) is brought together with its counterpart positron (with a positive charge), they destroy each other and are converted into energy.
Black hole: The remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.
Interstellar medium: The space between the stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.
Nuclear reaction: The processes by which an atomic nucleus is split (fission) or joined with another (fusion), resulting in the release of great amounts of energy.
Ozone layer: The atmospheric layer of approximately 15 to 30 miles (24 to 48 kilometers) above Earth's surface that protects the lower atmosphere from harmful solar radiation.
Pulsar: Rapidly rotating star that emits varying radio waves at precise intervals; also known as a neutron star because much of the matter within has been compressed into neutrons.
Quasar: Extremely bright, starlike sources of radio waves that are the oldest known objects in the universe.
Radiation: Energy emitted in the form of waves or particles.
Radioactivity: The property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.
Radio waves: Electromagnetic radiation, or energy emitted in the form of waves or particles.
Solar flare: Sudden outbursts of light extending from the Sun that last only five to ten minutes and produce an incredible amount of energy.
Subatomic particle: Basic unit of matter and energy smaller than an atom.
Supernova: The explosion of a massive star at the end of its lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.
Wavelength: The distance between one peak of a wave of light, heat, or energy and the next corresponding peak.
A major discovery of the CGRO was the class of objects called gamma-ray blazars, quasars that emit most of their energy as gamma rays and vary in brightness over a period of days. Scientists also have found evidence for the existence of antimatter based on the presence of gamma rays given off by the mutual destruction of electrons and positrons in the interstellar medium, or the space between the stars.
NASA decided to end the CGRO's mission after one of its three gyroscopes failed in December 1999. The observatory, which cost $670 million, could have been kept aloft for eleven more years, but NASA decided that if more equipment had failed, they could not control its return to Earth. So, on June 4, 2000, after completing 51,658 orbits of the planet, the CGRO re-entered Earth's atmosphere and broke apart. The charred remains of the craft—roughly 6 tons (5.4 metric tons) of superheated metal—splashed into the Pacific Ocean about 2,500 miles (4,020 kilometers) southeast of Hawaii.
To study gamma rays further, NASA plans to launch the Gamma Ray Large Area Telescope in 2005.